1. Field of the Invention
The present invention relates to a process for recovering tungsten carbide from cemented tungsten carbide scraps, and more particularly to a process for recovering tungsten carbide by selective electrolysis in which a chelating agent is added to the electrolyte for complexing with tungsten ions produced during electrolysis to prevent passivation of tungsten carbide.
2. Description of the Prior Art
Due to its superior hardness, cemented tungsten carbide has been extensively used in the manufacture of cutting tools, drill, dies, and abrasion-resistant mechanical components. Since tungsten is not abundant, process for its recovery have drawn researchers' concern in many countries. Statistics reveal that recovered tungsten carbide comprises about 20% to 30% of the total supply, lowering the raw material cost by about 15% to 50%.
To date two approaches have been used to recover tungsten carbide scrap. The first approach involves reacting tungsten carbide scrap to form an intermediate, such as oxide, by a physical or chemical method, followed by reducing the oxide to pure tungsten. Examples of such methods include the conventional nitrate-melt-leaching process, high temperature oxidation, phosphoric acid leaching, chlorination and high potential electrolysis. The other approach involves the dissolution of a cementing agent to obtain a fine powder of tungsten carbide which can be directly used. Such methods include the molten zinc method, the cold stream flow method and low potential electrolysis.
The most frequently applied techniques are the molten zinc method and the cold stream flow method. According to the molten zinc method, the tungsten carbide scrap is heated to 900.degree. C. with the introducton of argon gas, and is then vacuum distilled. The energy consumption is therefore great, about 4000 to 6000 kWh is required for one ton of tungsten carbide. The cold stream flow method includes heating cemented carbide scrap to a high temperature, injecting a high speed cold air flow to the scrap to break and separate the scrap, and then recovering tungsten carbide. This method also inevitably requires a lot of energy.
Recovering tungsten carbide from cemented carbides by electrolysis has been used since 1950. Such problems as complicated procedures and severe pollution encountered initially have been overcome recently with new developments in electrochemical technology. The electrolysis method has the following advantages: it saves energy, there is no pollution, the recovered product has high purity, and the investment for the equipment is low. For example, according to the high potential direct electrolysis method, tungsten carbide scrap, which serves as an anode, is electrolytically oxidized into anode slime of tungstic acid, and the cobalt cementing agent therein is thus dissolved in electrolyte to form cobalt ions. The anode slime of tungstic acid is then recovered by extraction or is reduced to tungsten metal. The cobalt can be recovered by electrolysis or by precipitation with the addition of oxalic acid. One disadvantage of this method is that the process for the recovery of tungsten carbide from anode slime of tungstic acid is complicated and therefore is not economical.
Nutzel and Kuhl in their EP 0 005 877 disclose a selective electrolysis process for recovering tungsten carbide from hard metal scrap containing the same. According to the process, the applied potential and energy consumption are low, the process and equipment for recovery are simple, and tungsten carbide can be directly recovered. According to Ghandehari, if a solution containing phosphoric acid is used as electrolyte in selective electrolysis for recovering tungsten carbide, the electrolysis can be carried out at a lower potential, and the efficiency for separating tungsten carbide and cobalt can be much improved (U.S. Pat. No. 4,234,333 and J. Electrochem. Soc. 127(1980):2144-2147). In technical papers by Dai Enzhong, "Reclamation of Cemented Carbide Scraps by Selective Electrolytic Dissolution Process (SEOP) at Recycling of Metalliferous Materials Conference, organized by the Institute of Mining & Metallurgy, 1990, 67-73, it is shown that tungsten carbide scrap will be subjected to passivation if the electrolysis is carried out in an acidic solution such as hydrochloric acid, nitric acid and sulfuric acid, thus causing the substantial reduction of dissolution rate. It is also indicated in these papers that if 2-3.5N HCl is used as electrolyte, and the electrolysis is carried out with the following conditions: tank voltage: 1.4-2.0 V, current density passing cathode: 140200 A/cm.sup.2, electrolyte temperature: 30.degree.-45.degree. C., and flow rate of electrolyte: 0.8-1.2 liter/cm.sup.2 min, the electrolytic efficiency can be increased substantially. However, all of the above mentioned selective electrolysis methods have a disadvantage, that is, the current will gradually decrease due to the passivation of anode during the electrolysis process at a fixed potential. Additional potential should be gradually increased for maintaining a constant current. This will substantially affect the electrolysis efficiency.